1. Field of the Invention
The present invention relates to a metal gasket composed of a folded portion and a sealing bead, and a process for manufacturing the metal gasket.
2. Related Art
As a metal gasket to be sandwiched between a cylinder block and a cylinder head of an engine, there have been proposed and practiced a variety of metal gaskets of a single-sheet or plural-sheet construction, which are made to regulate an abnormal deformation in a bead to be arranged at the outer circumference of a folded portion, by forming an opening in registration with a cylinder bore, and by forming the folded portion at the edge of the opening to retain a clearance as thick as the folded portion at the outer circumference of the folded portion.
However, the folded portion is subjected at its edge to a repeated load in the thickness direction (as will be called the "compressing direction") of the metal gasket when the engine is run, because it is sandwiched between the cylinder head and the cylinder block, and is subjected to a repeated load in the facial direction (as will be called the "shearing direction") of the metal gasket due to the temperature change from the difference in the coefficient of thermal expansion between the cylinder head and the cylinder block. As a result, as shown in FIG. 32, there arises a problem that cracks 101 penetrate into a folded edge 100 from the inside to break a folded portion 102.
In the prior art, therefore, the mainstreams of the prior art are given plural-sheet constructions in which the metal gasket is composed of two or more gasket sheets. In one mainstream construction, a folded portion is so formed in the second gasket sheet as to embrace the opening edge of the first gasket sheet thereby to enlarge its radius of curvature so that the cracks at the folded edge are prevented. In another mainstream construction, the cracks at the folded edge are prevented by making the first gasket sheet of a material having a little elongation but a high tensile strength to have excellent spring properties, by making the second gasket sheet of a material having a low tensile strength but a large elongation, by forming a bead in the first gasket sheet to retain a sufficient bead performance and by forming the folded portion in the second gasket sheet.
In Unexamined Published Japanese Patent Application No. 5-65959, moreover, there has also been proposed a metal gasket which is constructed to suppress the cracks in the folded portion by forming an annular groove in a position corresponding to the inner face of a folded edge before the folded portion is formed in an elastic metal sheet for the gasket sheet, to enlarge the radius of curvature of the folded edge.
As the beads in the gasket sheet, on the other hand, there are formed the rounded beads or stepped beads enclosing a combustion chamber, oil holes, bolt holes and outer circumferences. These rounded beads and stepped beads are compressed and deformed between the cylinder block and the cylinder head to establish the desired sealing performance.
Here, the compressively deforming force to act upon the beads is established by a plurality of head fixing bolts to be fastened into the cylinder block. As a result, if the sectional shape and size of the beads are made identical, the bearing stress to act on the beads is made the lower at the more distance from the head fixing bolts by the deflection of the cylinder block or the cylinder head. This raises a problem that the sealing performance drops in that portion. On the other hand, the bearing stress rises in the portions, in which a plurality of beads are arranged close to each other, or in the portions, in which a plurality of beads intersect each other, whereas the bearing stress at the bead portions other than those portions become lower to raise a problem that the sealing performance also drops partially.
In order to solve these problems, it is conceivable to lower or widen the beads at the portions in which the bearing stress rises. However, the method of changing the height of the beads is required to change the height of the bead forming portions in a mold structure for molding the beads. As a result, the mold structure is complicated to raise the cost for producing the mold itself and to make it liable for an offset wear to occur thereby to make the mold maintenance troublesome. It is, therefore, a current practice to adopt a method of adjusting the bearing stress by changing the widths of the beads.
Of the beads for sealing the combustion chamber and the beads for sealing the outer periphery of the metal gasket, on the other hand, the former beads for sealing the combustion gas under a high pressure are more strictly demanded for the sealing performance. It is, therefore, a current practice to employ the rounded beads as the former ones and the stepped beads as the latter ones. As a result, the bearing stress of the beads around the combustion chamber is set to a higher level than that of the beads of the outer periphery thereby to provide a sufficient sealing performance.
Moreover, there has been widely adopted in the prior art a technique for raising the hardness of beads as a whole by subjecting the entire gasket sheet to a heat treatment.
The metal gasket of the prior art is basically constructed to prevent the cracks at the folded edge by adopting the method of enlarging the radius of curvature of the folded edge and the method of employing a material having a large elongation for the gasket sheet to form the folded portion. For the former method, it is difficult to reduce the cracks sufficiently at the folded edge. For the latter method, it is impossible to manufacture a metal gasket having a single-sheet construction, but an expensive material having an excellent elongation has to be used to raise a problem that the production cost is raised.
We have been able to attain the following concept on the mechanism for the cracks to occur at the folded edge. This concept has been attained by noting that the hardness of the vicinity of the folded portion is caused, when the elastic metal sheet is folded, by the hardening action of the folding operation to take a numerical value, as shown in FIG. 33, for SUS430 2B so that the hardness at the folded edge becomes higher than that of the remaining portions to invite the cracks. By this notation, the hardness of only the vicinity of the folded edge is lowered to ensure its sufficient elongation so that the folded edge can be freed from the cracks while retaining sufficient spring characteristics at the remaining portions.
As to the bead portions, on the other hand, it may be impossible from the restrictions on the space to improve the sealing performances by changing the width of the beads, as described above. In the portions in which a plurality of beads are arranged close to each other, more specifically, the beads may be unable, if wide, to be arranged from the restrictions on the space. Moreover, the restrictions on the space become the more severe in accordance with the smaller size of the engine of recent years. The result at the present stage is that the design is compelled to sacrifice the sealing performance to some extent.
We have found out that the bearing stress to act on the beads can be adjusted to a low level by lowering the hardness of the portion, the bearing stress of which is to be lowered, like before to lower the spring constant of the beads without changing the width of the beads.